The reinforcing armature or reinforcement of the tires and in particular of motorcycle tires is currently—and more often than not—formed by the stacking of one or more plies conventionally designated “casing plies”, “breaker strips”, etc. This method of designating the reinforcing armatures originates from the manufacturing method, consisting in producing a series of semi-finished products in the form of plies, provided with wire reinforcement, often longitudinal, which are then assembled or stacked in order to form a tire blank. The plies are produced flat, with large dimensions, and are then cut to the dimensions of a given product. The plies are also initially assembled substantially flat. The blank produced in this way is then shaped to adopt the typical toroidal profile of the tires. The semi-finished products, called “finishing” products, are then applied to the blank, to obtain a product ready for vulcanization.
Such a “conventional” type of method involves, in particular for the tire blank manufacturing phase, the use of an anchoring element (generally a bead wire), used to provide the anchorage or secure the carcass armature within the area of the beads of the tire. Thus, for this type of method, a portion of all of the plies forming the carcass armature (or a part only) is turned up about a bead wire arranged in the bead of the tire. In this way, an anchoring for the carcass armature is created in the bead.
The roll-out in industry of this type of conventional method, despite numerous variants in how to produce the plies and the assemblies, has led those skilled in the art to use a vocabulary suited to the method; hence the generally accepted terminology, notably including the terms “plies”, “carcass”, “bead wire”, “conformation” to designate the transition from a flat profile to a toroidal profile, etc.
There are, today, tires that do not genuinely include “plies” or “bead wires” according to the above definitions. For example, the document EP 0 582 196 describes tires manufactured without the use of semi-finished products in the form of plies. For example, the reinforcing elements of the different reinforcement structures are applied directly to the adjacent layers of rubbery mixtures, the whole being applied in successive layers to a toroidal core, the shape of which is such that a profile is directly obtained that is similar to the final profile of the tire being manufactured. Thus, in this case, there are no longer “semi-finished” products, or “plies” or “bead wires”. The basic products, such as the rubbery mixtures and the reinforcing elements in the form of wires or filaments, are directly applied to the core. Since this core is toroidal in shape, there is no longer a need to form the blank to change from a flat profile to a profile in the form of a torus.
Moreover, the tires described in this document do not have the “traditional” casing ply turn-up about a bead wire. This type of anchoring is replaced by an arrangement in which circumferential wires are positioned adjacent to said sidewall reinforcement structure, the whole being embedded in a rubbery anchor or binding mixture.
There are also methods of assembly on a toroidal core that use semi-finished products that are specially adapted for rapid, effective and simple application on a central core. Finally, it is possible to use a mix comprising both certain semi-finished products to produce certain architectural aspects (such as plies, bead wires, etc.), whereas others are produced from the direct application of mixtures and/or reinforcing elements.
In the present document, in order to take account of the recent technological trends both in the manufacturing domain and in the design of products, the conventional terms such as “plies”, “bead wires”, etc., are advantageously replaced by neutral terms or terms that are independent of the type of method used. Thus, the term “carcass-type reinforcement” or “sidewall reinforcement” is valid to designate the reinforcing elements of a casing ply in the conventional method, and the corresponding reinforcing elements, generally applied to the sidewalls, of a tire produced according to a method without semi-finished products. The term “anchorage area”, for its part, can designate both the “traditional” casing ply turn-up about a bead wire of a conventional method, and the assembly formed by the circumferential reinforcing elements, the rubbery mixture and the adjacent sidewall reinforcing portions of a bottom area produced with a method with application on a toroidal core.
As in the case of all the other tires, we are witnessing a radialization of the tires for motorbikes, the architecture of such tires comprising a carcass armature formed by one or two layers of reinforcing elements that form with the circumferential direction an angle than can be between 65° and 90°, said carcass armature being radially topped by a crown armature comprising at least reinforcing elements that are generally textile. There are, however, non-radial tires to which the invention also relates. The invention even relates to partially radial tires, that is, tires in which the reinforcing elements of the carcass armature are radial over at least a part of said carcass armature, for example in the part corresponding to the crown of the tire.
Numerous crown armature architectures have been proposed, depending on whether the tire is intended to be mounted on the front of the motorbike or on the back. A first structure consists, for said crown armature, in employing only circumferential ropes, and said structure is more particularly used for the rear position. A second structure, directly inspired by the structures commonly employed in tires for private passenger vehicles, has been used to improve the wear resistance, and consists in the use of at least two crown working layers of reinforcing elements parallel with each other in each layer but crossed from one layer to the next, forming with the circumferential direction acute angles, such tires being more particularly suited to the front of the motorbikes. Said two crown working layers can be associated with at least one layer of circumferential elements, generally obtained by helically winding a strip of at least one rubber-coated reinforcing element.
The patent FR 2 561 588 thus describes such a crown armature, with at least one ply, the reinforcing elements of which form, with the circumferential direction, an angle that can vary between 0° and 8°, the modulus of elasticity of such elements rising to at least 6000 N/mm2, and, arranged between the carcass armature and the ply of circumferential elements, a damping layer mainly formed by two plies of elements crossed from ply to the next, forming between them angles of between 60° and 90°, said crossed plies being formed by textile reinforcing elements having a modulus of elasticity of at least 6000 N/mm2.
The document EP 0 456 933, in order to give a motorbike tire excellent stability at high speed and an excellent ground contact property, discloses, for example, forming a crown armature with at least two plies: a first ply, radially closest to the carcass armature, consisting of ropes oriented with an angle of between 40° and 90° relative to the circumferential direction and the second ply, radially closest to the tread, consisting of ropes wound helically in the circumferential direction.
The U.S. Pat. No. 5,301,730, in order to increase the driveability of a tire for the rear position of a motorbike, proposes a crown armature consisting, working from the radial carcass armature to the tread, of at least one ply of substantially circumferential elements and two plies of elements crossed from one ply to the next, forming with the circumferential direction an angle that can be between 35° and 55°, the ply of elements parallel to the circumferential direction being able to be formed by elements made of aromatic polyamide, and the plies of crossed elements being able to be made of aliphatic polyamide.
In their researches, the inventors have shown that, to increase the transmission of the engine and braking torques and the lateral thrust forces via the tires, one solution is to run at pressures lower than those usually prescribed and in particular at pressures less than 2 bar, even less than 1.5 bar. The usual pressures are greater than 2 bar, even greater than 2.5 bar.
Running tests carried out in this way have confirmed the increase in the transmission of the engine and braking torques and of the lateral thrust forces through the tires; however, it emerges that, at these tire inflation pressures, the motorbike rider observes behavior problems and in particular perception problems according to the lateral direction of the tire, that is, according to the direction perpendicular in the plane of the ground to the direction defined by the two geometrical centre points of areas of contact of each of the tires.